diff --git a/lib/__tests__/calendar-ics-export.test.ts b/lib/__tests__/calendar-ics-export.test.ts
new file mode 100644
index 000000000..d684938c6
--- /dev/null
+++ b/lib/__tests__/calendar-ics-export.test.ts
@@ -0,0 +1,55 @@
+import { describe, expect, it } from 'vitest';
+import { eventToICS } from '@/lib/calendar-ics-export';
+import type { CalendarEvent } from '@/lib/jmap/types';
+
+function event(overrides: Partial<CalendarEvent> = {}): CalendarEvent {
+  return {
+    id: 'e1', uid: 'export-test@example.com', title: 'Sync', start: '2026-11-20T09:00:00',
+    timeZone: 'Europe/Berlin', duration: 'PT1H', showWithoutTime: false, calendarIds: { c: true },
+    ...overrides,
+  } as CalendarEvent;
+}
+
+// As the downloaded file holds it: UTF-8, where half an emoji on each side
+// of a fold turns into two U+FFFD.
+const asFile = (ics: string) => new TextDecoder().decode(new TextEncoder().encode(ics));
+const unfold = (ics: string) => asFile(ics).replace(/\r\n /g, '');
+const line = (ics: string, name: string) => unfold(ics).split('\r\n').find((l) => l.startsWith(name)) ?? '';
+
+describe('eventToICS', () => {
+  // Stalwart 0.16 and 1.0 both read a title folded inside an emoji back
+  // with two U+FFFD in its place.
+  it('folds lines between characters, at most 75 octets long', () => {
+    const title = `${'A'.repeat(64)} 🎉🎉🎉 Launch party 🎉 ${'Ünïcödé '.repeat(12)}`;
+    const ics = eventToICS(event({ title }));
+
+    expect(line(ics, 'SUMMARY:')).toBe(`SUMMARY:${title}`);
+    for (const physical of ics.split('\r\n')) {
+      expect(new TextEncoder().encode(physical).length).toBeLessThanOrEqual(75);
+    }
+  });
+
+  // A colon in an unquoted CN ended the parameters there: both servers read
+  // the organizer's address as " Alice:mailto:...".
+  it('quotes participant names that hold a colon, semicolon or comma', () => {
+    const ics = eventToICS(event({
+      participants: {
+        o: { '@type': 'Participant', email: 'alice@example.com', name: 'Ops: Alice', roles: { owner: true } },
+        p: { '@type': 'Participant', email: 'john@example.com', name: 'Doe, John; "JD"', roles: { attendee: true } },
+      } as unknown as CalendarEvent['participants'],
+    }));
+
+    expect(line(ics, 'ORGANIZER')).toBe('ORGANIZER;CN="Ops: Alice":mailto:alice@example.com');
+    expect(line(ics, 'ATTENDEE')).toBe('ATTENDEE;CN="Doe, John; \'JD\'";PARTSTAT=NEEDS-ACTION:mailto:john@example.com');
+  });
+
+  it('leaves a plain name unquoted', () => {
+    const ics = eventToICS(event({
+      participants: {
+        o: { '@type': 'Participant', email: 'alice@example.com', name: 'Alice', roles: { owner: true } },
+      } as unknown as CalendarEvent['participants'],
+    }));
+
+    expect(line(ics, 'ORGANIZER')).toBe('ORGANIZER;CN=Alice:mailto:alice@example.com');
+  });
+});
diff --git a/lib/calendar-ics-export.ts b/lib/calendar-ics-export.ts
index 1f13e0bb1..83c1c0f2e 100644
--- a/lib/calendar-ics-export.ts
+++ b/lib/calendar-ics-export.ts
@@ -1,15 +1,26 @@
 import type { CalendarEvent } from "@/lib/jmap/types";
 
-const MAX_LINE_OCTETS = 74;
+const MAX_LINE_OCTETS = 75;
+const utf8 = new TextEncoder();
 
+// RFC 5545 §3.1: lines of at most 75 octets, folded between characters. Cut
+// by UTF-16 units, a fold could split an emoji, which the file then held as
+// two U+FFFD.
 function foldLine(line: string): string {
-  if (line.length <= MAX_LINE_OCTETS) return line;
-  const chunks: string[] = [line.slice(0, MAX_LINE_OCTETS)];
-  let pos = MAX_LINE_OCTETS;
-  while (pos < line.length) {
-    chunks.push(" " + line.slice(pos, pos + MAX_LINE_OCTETS - 1));
-    pos += MAX_LINE_OCTETS - 1;
+  const chunks: string[] = [];
+  let chunk = "";
+  let octets = 0;
+  for (const char of line) {
+    const size = utf8.encode(char).length;
+    if (octets + size > MAX_LINE_OCTETS) {
+      chunks.push(chunk);
+      chunk = " ";
+      octets = 1;
+    }
+    chunk += char;
+    octets += size;
   }
+  chunks.push(chunk);
   return chunks.join("\r\n");
 }
 
@@ -22,6 +33,14 @@ function escapeText(value: string): string {
     .replace(/\r\n|\r|\n/g, "\\n");
 }
 
+// RFC 5545 §3.2: a parameter value holding ":", ";" or "," is quoted, and
+// can't hold a DQUOTE or a line break. Unquoted, "CN=Ops: Alice" ended the
+// parameters at the colon.
+function paramValue(value: string): string {
+  const clean = value.replace(/"/g, "'").replace(/[\r\n]+/g, " ");
+  return /[:;,]/.test(clean) ? `"${clean}"` : clean;
+}
+
 function stripDateSeparators(value: string): string {
   return value.replace(/[-:]/g, "").replace(/\.\d{3}/, "");
 }
@@ -142,7 +161,7 @@ export function eventToICS(event: CalendarEvent): string {
     if (organizer) {
       const email = organizer.email || organizer.sendTo?.imip?.replace("mailto:", "");
       if (email) {
-        const cn = organizer.name ? `;CN=${escapeText(organizer.name)}` : "";
+        const cn = organizer.name ? `;CN=${paramValue(organizer.name)}` : "";
         lines.push(`ORGANIZER${cn}:mailto:${email}`);
       }
     }
@@ -150,7 +169,7 @@ export function eventToICS(event: CalendarEvent): string {
       if (p.roles?.owner) continue;
       const email = p.email || p.sendTo?.imip?.replace("mailto:", "");
       if (!email) continue;
-      const cn = p.name ? `;CN=${escapeText(p.name)}` : "";
+      const cn = p.name ? `;CN=${paramValue(p.name)}` : "";
       const partstat = p.participationStatus
         ? `;PARTSTAT=${p.participationStatus.toUpperCase()}`
         : ";PARTSTAT=NEEDS-ACTION";